Tonino Picula, the head of an observer mission run by the Organisation for Security and Co-operation in Europe, said the result was never in doubt and the vote had offered no genuine competition.

"The point of elections is that the outcome should be uncertain. This was not the case in Russia," he said.

According to the central elections commission, Putin won nearly 64% of the vote. Opposition leaders have denounced the result as "illegitimate" and have promised to bring thousands of people onto the streets to protest on Monday evening.

Their anger has been fuelled by widespread reports of fraud, including evidence of ballot-stuffing and "carousel voting", when voters are employed to cast their votes several times at various polling sites.

The OSCE said that while the process of voting was "assessed positively overall", problems began after the polls closed, with irregularities registered at almost one-third of polling stations during the count. They called for all irregularities to be investigated.

"There was no real competition and abuse of government resources ensured that the ultimate winner of the election was never in doubt."

Putin's campaign chief has called the election the cleanest in Russia's history. Announcing his victory on Sunday night, Putin said he had won "an open and honest battle".

Vladimir Churov, the head of the Russian election commission, built on Putin's rhetoric that the opposition protests that have rocked Russia since a similarly contested parliamentary vote in December were being directed from the west.

Denouncing foreign election missions on Monday, Churov said some observers were looking for "military and political information" under the guise of seeking electoral violations.

"More and more often, monitors feel the irresistible desire to penetrate border zones, closed nuclear sites, missile centres and so on," he said on Monday.

The Kremlin deployed around 12,000 police and an extra 6,300 interior ministry troops around the capital again on Monday. More radical elements inside the protest movement have called on people to set up a tent camp. The Kremlin has said it will not allow any "provocations", and pro-Kremlin youth activists were also gearing up to gather in the evening.

As results poured in from around the country, it became clear that Putin had lost his grip on the capital. He failed to break the 50% barrier in Moscow, taking 47% of votes.

Opposition activists said this still appeared too high, and cited evidence of vote-rigging. The billionaire challenger Mikhail Prokhorov, who has battled claims that his relatively liberal candidacy was a Kremlin project to deflect protest anger, took second place in the capital with just over 20%. Outside Moscow, the Communist leader, Gennady Zyuganov, came in a distant second, as had been expected.

"Putin has named himself the emperor of Russia for the next 12 years," Alexey Navalny, a protest leader, said on Sunday. "We announced earlier that we will not recognise these elections. The powers here are illegitimate."

He also told the Russian justice minister to explain why the liberal opposition Parnas movement had been refused party registration.

The opposition shrugged off the move. "It's clear that Medvedev wants to leave good memories of himself and to demonstrate his commitment to the truth," the Parnas leader and former prime minister Mikhail Kasyanov told Interfax.

"The main task has been fulfilled – Putin has returned to the Kremlin, and it's again time for Medvedev to carry out an imitation of liberal tendencies in political life."